Low-Temperature Anomaly in the Penetration Depth ofYBa2Cu3O7Films: Evidence for Andreev Bound States at Surfaces

Abstract

We report the observation of anomalous Meissner currents in thin films of superconducting YBCO with oriented internal surfaces introduced by heavy-ion bombardment. High-precision measurements of the penetration depth ( $\ensuremath{\lambda}$) reveal an upturn in the temperature dependence of $\ensuremath{\lambda}$ below ${T}^{*}\ensuremath{\approx}15\mathrm{K}$. The magnitude of the observed effect, its onset temperature, and its dependence on the orientation of the surfaces are in quantitative agreement with the theory of surface effects in $d$-wave superconductors. The anomaly is interpreted to arise from surface currents that are carried by surface-induced Andreev bound states.